1. Field of the Invention
The present invention relates to an industrial robot using a parallel link mechanism.
2. Background Art
A known industrial robot using a parallel link mechanism (hereafter called “robot”) is disclosed in Japanese Translation of PCT Application No. 2002-532269. This robot has a base unit, which is a support base, a head unit (end effector) including various tools or the like (hereafter called “tool”), and a plurality (three) of arms which respectively connect the base unit to the head unit, and the position and orientation of the head are changed by driving each arm individually in a collaborative manner using a motor installed in the base unit. This robot has a rotating shaft that is separate from the arms and that connects the motor installed in the base unit to the tool, whereby the tool can be rotary-driven by the motor. The rotating shaft is retractable, and both ends thereof are connected to the tool and the motor via a universal joint respectively. Therefore the rotating shaft can transfer the rotary driving force of the motor to the head, while following up the changes of the position and orientation of the head.
In concrete terms, the rotating shaft is configured by a cylindrical outer casing, an inner shaft, and a bushing which is secured inside the outer casing and holds the inner shaft with respect to the outer casing, so that the inner shaft can freely move in the shaft direction. In other words, the rotating shaft can be retractable by the inner shaft and the outer casing relatively displacing in the shaft direction. The inner shaft and the busing are configured by the so-called ball spline. The ball spline includes a spline shaft, a tubular unit in which the spline shaft is inserted, and a plurality of balls which is supported by the tubular unit and exists between the tubular unit and the spline shaft, and the spline shaft and the tubular unit are combined via the balls so that the spline shaft can be relatively displaced with respect to the tubular unit in the shaft direction, and the relative rotation of the spline shaft with respect to the tubular unit is constrained. In other words, in the rotating shaft, the inner shaft is constituted by the spline shaft, and the bushing is constituted by the tubular unit.
The rotating shaft which is applied to the robot according to Japanese Translation of PCT Application No. 2002-532269 has the following problems. In other words, the robot is demanded to operate at high-speed, and the arms and the rotating shaft must be light to implement the high-speed operation. However in the case of the configuration of the rotating shaft using the ball spline described above, each ball existing between the inner shaft (spline shaft) and the bushing (tubular unit) contacts the inner shaft and the bushing as point contacts. Therefore if the diameters of the inner shaft and the bushing are decreased to make the rotating shaft lighter, the contact pressure decreases since the diameter of the ball becomes smaller accordingly, which makes it difficult to transfer a desired running torque. This means that decreasing the size and the weight of the rotating shaft is difficult.
Furthermore, lubricating oil is normally supplied to the balls in order to slide the inner shaft for the bushing smoothly. But if a conventional rotating shaft is used, the inner shaft on which the lubricating oil adheres is exposed outside the outer casing when the rotating shaft is extended, which means that lubricating oil may splash. Therefore it is difficult to use the robot in a cleanroom, or to use the robot in food related application fields.